What Are Antibiotics? A Complete Guide to Uses, Types, Safety, and Prescription Requirements

By the A2Z RX LLC Editorial Team | Reviewed for accuracy by a licensed pharmacist | Last reviewed: [

29 September 2026]

Quick Q&A: What Are Antibiotics?

Q: What are antibiotics?
A: Antibiotics are prescription medicines that kill bacteria or stop them from multiplying, allowing your immune system to clear a bacterial infection.

Q: Do they work on colds and flu?
A: No. Colds, flu and COVID-19 are caused by viruses, and antibiotics have no effect on them.

Q: Do I need a prescription?
A: Yes. In the United States, oral antibiotics require a valid prescription from a licensed prescriber.

Q: What is the golden rule?
A: Take exactly what is prescribed, for the full duration, and never share or save leftover doses.

What Are Antibiotics and Why Do They Matter?

If you have ever asked yourself What Are Antibiotics, the short answer is that they are medicines designed to fight infections caused by bacteria. Since penicillin entered clinical use in the 1940s, antibiotics have turned once-deadly conditions such as pneumonia, sepsis, and strep throat complications into treatable illnesses. Understanding how they work, and just as importantly when they do not, helps you use them safely and protects their effectiveness for everyone.

How Do Antibiotics Work?

Bacteria are single-celled organisms. Most are harmless or even helpful, but some invade the body and cause illness. Antibiotics act in one of two ways:

• Bactericidal antibiotics kill bacteria directly, often by breaking down the cell wall so the organism bursts. Penicillins and cephalosporins work this way.
• Bacteriostatic antibiotics stop bacteria from growing or reproducing, giving your immune system time to finish the job. Tetracyclines and macrolides are common examples.

Because human cells do not have bacterial cell walls, well-chosen antibiotics can target the germ while leaving your own cells largely unharmed. That selectivity is why these medicines are so valuable, and why they must be matched to the right bacteria.

Antibiotics vs. Viruses: A Critical Difference

Viruses are structurally different from bacteria and reproduce inside your own cells, so antibiotics cannot reach them. Taking an antibiotic for a viral illness will not shorten it, will not prevent you from spreading it, and may cause side effects and contribute to resistance. According to the U.S. Centers for Disease Control and Prevention (CDC), a significant share of outpatient antibiotic prescriptions in the U.S. are unnecessary, which is why a professional evaluation matters before starting treatment.

Common Conditions Treated With Antibiotics

Prescribers use antibiotics for bacterial infections that have been diagnosed or strongly suspected based on symptoms, examination or lab testing. Typical examples include:

• Strep throat and bacterial sinus infections
• Middle ear infections, especially in children
• Urinary tract infections (UTIs)
• Bacterial pneumonia and bronchitis complications
• Skin and soft-tissue infections such as cellulitis
• Dental infections and abscesses
• Certain sexually transmitted bacterial infections

Each infection responds best to a specific drug, dose, and duration, so antibiotics are never interchangeable.

Major Types (Classes) of Antibiotics

Antibiotics are grouped into classes by how they work and which bacteria they target. The table below summarizes the most commonly prescribed classes in the United States.

Amoxicillin: One of the Most Prescribed Antibiotics

Amoxicillin is a penicillin-class antibiotic valued for its broad coverage, long track record, and generally good tolerability. It is available in capsules, tablets, chewables and liquid suspension, with common strengths of 250 mg, 500 mg and 875 mg. Your prescriber chooses the strength, frequency, and duration based on the infection, your age, weight, kidney function, and allergy history.